1. Field of the Invention
The present invention relates to a thermal transfer printer which is used as an output device such as typewriters, word processors, computers, etc., and more particularly to an improvement in a peel angle of an ink ribbon.
2. Description of the Prior Art
Generally, a theremal transfer printer has a columnar support shaft 3 extending lengthwise of a platen 1, the shaft 3 being located frontwardly and downwardly of the platen 1 having a platen rubber 2 lined on the front surface thereof, as shown in FIG. 5. A carriage 4 which is movable lengthwise of the platen 1 is disposed along the support shaft 3. A ribbon cassette 5 encasing therein an ink ribbon 6 is placed on the carriage 4, and a thermal head 8 supported on a head mounting member 7 is disposed movably lengthwise of the platen 1 integral with the carriage 4. The head mounting member 7 is swingingly supported on the support shaft 3, and a rearwardly projecting guide plate 9 is provided on the mounting member 7. A ]-shaped portion 12 at the end of a rotating shaft 11 supported swingingly on a pin 10 is relatively movably fitted in the rear end of the guide plate 9. One end of a spring 13 is mounted on the rotating shaft 11 so that the head mounting member 7 is always urged by means of the spring 13 in the direction where the thermal head 8 is pressed against the platen 1 through the rotating shaft 11 and the guide plate 9. A solenoid 14 is connected to the rotating shaft 11 so that when the solenoid 14 is driven, the head mounting member 7 is rotated clockwise in FIG. 5 against the force of the spring 13 to relase the thermal head 8 from the connection with the platen 1.
FIG. 6 shows a specifica example of the ribbon cassette 5. The ink ribbon 6 encased in the ribbon cassette 5 has its middle portion guided outside from a ribbon take-out portion 5A formed in the front surface of the ribbon cassette 5, and the ink ribbon 6 pisitioned thereat is pulled into the ribbon cassette 5 from a ribbon pull-in portion 5B. The ink ribbon 6 is moved by rotation of a rotating drum 15 in a direction as indicated by the arrow. The ribbon cassette 5 at the back of the ink ribbon 6 is further formed with a recess 16, to which the thermal head 8 faces.
In the conventional thermal transfer printer, as shown in FIGS. 7 and 8, the carriage 4 moved along the support shaft 3 by means of a wire 17 is formed with a recess 18 in which the head mounting member 7 is fitted. The head mounting member 7 is moved within the recess 18 in a direction to and from the platen 1 by the action of the spring 13 and solenoid 14 shown in FIG. 5.
With the above-described construction, when the solenoid 14 stops to be driven, the head mounting member 7 is swung counterclockwise in FIG. 5 by the action of the spring 13, and the mounting member 7 of FIG. 7 to the state where the former is positioned within the recess 18 to the state where the head mounting member 7 of FIG. 8 is projected toward the platen 1, and at that time, the thermal head 8 clamps the ink ribbon 6 and a record paper 19 (FIG. 5) between it and the platen 1, whereby ink at a predetermined position of the ink ribbon 6 is transferred onto the paper 9 by heat of the thermal head 8.
However, in the aforementioned conventional thermal transfer printer, as shown in FIG. 9, in the state where the thermal head 8 is pressed against the platen 1 through the ink ribbon 6 and paper 19, an angle .theta. formed between the end of the thermal head 8 and the ribbon pull-in portion 5B of the ribbon cassette 5, that is, the peel angle .theta. of the ink ribbon 6 is given by EQU .theta.=tan.sup.-1 (a/b)
where a represents the distance from the front surface of the thermal head 8 to the projection 20 of the ribbon cassette 5, and b represents the distance from the end of the thermal head 8 to the pull-in portion 5B of the ribbon cassette 5. This angle .theta. becomes a relatively samll angle because of the presence of wall thickness c of teh ribbon cassette 5.
If the peel angle .theta. is small as described above, when the ink ribbon 6 is heated by the thermal head 8 to transfer ink of the ink ribbon 6 onto the paper 19, ink becomes transferred also onto the paper 19 in the outer peripheray of a portion to be printed, and as a result, the print becomes collapsed and hard to see.